perhaps someone impersonated you and got those UC benefits. The place to start is with the UC bureau to make sure it was really you that got the benefits. if it was not then send back the letter to the IRS with a letter of explanation that you were a victim of identity theft and include form 14039.
https://www.irs.gov/pub/irs-pdf/f14039.pdf
if it was you pay up. you owe the taxes and the IRS probably won't abate penalties unless you can provide a reasonable cause. Not getting the 1099-G is not reasonable cause because you should have been aware that you got the money.
